1. Field of the Invention
The present invention relates to a method and apparatus for displaying an X-ray image which detect in the form of an electric signal an X-ray image relating to an intraoral region or the like of a subject so that the X-ray image is displayed on a CRT (cathode ray tube) or the like.
2. Description of the Related Art
Conventionally, in order to obtain an X-ray image of an intraoral region, a film method is widely employed in which a photosensitive recording material such as a silver film is exposed to an X-ray followed by developing and fixing processes.
However, the film method has problems in that: 1) a time period of about 2 minutes or longer must be required between the X-ray exposure and the observation of the X-ray image; 2) a developing apparatus and processing solutions for conducting the developing and fixing processes are indispensable; 3) the X-ray sensitivity of a silver salt has a limitation; therefore, an X-ray dose of a predetermined level is required to obtain a desired image density; and 4) it is impossible to correct an image which has been once fixed.
In order to solve these problems, an X-ray imaging apparatus is proposed in which an X-ray image is converted into an electric signal by using an imaging device such as a CCD (charge coupled device) and the X-ray image is then displayed on a CRT (cathode ray tube) or the like. The X-ray imaging apparatus employs a so-called non-film which does not use a photosensitive recording material such as a silver film, and has features in that: 1) an X-ray image can be observed in real time after the exposure of X-rays; 2) a developing apparatus and processing solutions are entirely unnecessary; 3) the X-ray sensitivity characteristic of an imaging device is linear and therefore, an X-ray dose can be reduced; and 4) a detected X-ray image can be subjected to various image processings, as well as can be easily copied or stored.
In contrast to an X-ray photograph based on a silver film having a density range of 0 to 4, an apparatus for displaying a digital image, such as a CRT or a thermal printer has a low density range of 0 to 2. With respect to a density gradient, furthermore, 64 gradients can be distinguished at the maximum by the naked eye.
Since a silver film is subjected to observation of the transmitted light due to the silver grain masking, it is excellent in brightness contrast. In contrast, a CRT operates on the basis of the visible-light conversion of a fluorescent material impinged by an electron beam, and therefore, a CRT is low in brightness, and has an inferior dynamic range. A thermal printer which utilizes a black degree appearing on thermosensitive papers is subjected to the reflection light observation, and hence, it is not excellent in brightness contrast and gradient resolution. Consequently, radiologists and doctors who are familiar with diagnosis based on an image formed on a silver film show a tendency to evaluate that a CRT and a thermal printer are inferior in the amount of information provided.
In order to solve the lack of information, it is proposed that an X-ray image is represented by a digital signal of 8 bits (256 levels) or 12 bits (1,024 levels), and is subjected to various kinds of image processing such as the gradient conversion, the integration image, the shading correction, the difference input, the edge enhancement, the addition input, the convolute processing, the filtering, and the inter-image operation, thereby improving the diagnosis ability.
In a medical diagnosis, the gradient conversion is particularly important. However, the gradient conversion has a problem in that, when the gradient emphasis is linearly conducted on a specific density region, the region may be converted into an image which is easy to see but densities of other regions jump to an extreme level, or the entirely black level or the entirely white level, whereby it is made difficult to diagnose the whole region. Alternatively, a multi-stage emphasizing technique may be employed in which the whole density range is divided into plural subregions and the gradient emphasis is conducted discontinuously on the subregions. According to this technique, the density can be prevented from jumping to the entirely black level or the entirely white level, but equidensity lines appear in an image in a manner similar to contour lines used in a map. This causes the diagnosis to be very difficult to conduct.